The present invention relates to a structure for mounting an exhaust manifold to the body of an internal combustion engine and, more particularly, to a mounting structure for an exhaust manifold having a plurality of branch tubular portions and a continuous strip-like flange portion which joins the free end portions of said branch tubular portions.
The exhaust manifold of a multi-cylinder internal combustion engine employed in an automobile is heated up to a relatively high temperature such as 500.degree.-600.degree. C. by hot exhaust gases flowing therethrough during operation of the engine, whereby the exhaust manifold thermally expands to elongate in the lengthwise direction of the multi-cylinder engine as well as to bend relative to the side wall of the body of the engine to which the manifold is mounted, because the flange portion of the manifold is cooled by the cylinder head and is maintained at a moderate temperature, whereas the joining portions of the tubular portions are heated up to a high temperature by exhaust gases. Conventionally, an exhaust manifold, particularly an exhaust manifold for a multi-cylinder engine having a plurality of branch tubular portions and a continuous strip-like flange portion which joins the free end portions of said branch tubular portions, is firmly fastened to a side wall portion of the body of an internal combustion engine by a plurality of mounting bolts arranged substantially uniformly over the entire length of the strip-like flange portion, with the interposition of a gasket sheet means therebetween. In this mounting structure, it is often contemplated to relieve thermal expansion of the manifold in the lengthwise direction of the engine by incorporating a shiftable structure at the bolt mounting portions, such as elongated bolt holes, whereby such lengthwise expansion of the manifold is relatively easily relieved without causing any leakage clearance between the exhaust manifold and the body of the engine. However, with regard to the aforementioned bending of the exhaust manifold, no means has yet been proposed which can effectively relieve such a bending without causing any leakage clearance between the exhaust manifold and the body of the engine. Therefore, in the conventional structure for mounting an exhaust manifold, particularly a manifold for a multi-cylinder engine having a strip-like flange portion, to the body of an internal combustion engine, the strip-like flange portion is firmly fastened over its entire length by a number of mounting bolts so as forcibly to suppress the bending of the exhaust manifold thereby maintaining the flange portion of the manifold substantially in contact with the cooperating side wall portion of the body of the engine over the entire region thereof even in the hot operating condition. However, when the thermal bending of the exhaust manifold is restricted in the abovementioned manner, the central portion between the opposite ends of the manifold is compressed under high thermal stressing during hot operation and undergoes plastic deformation (compression), whereby the shape of the exhaust manifold in its normal unheated condition becomes inversely bent and begins to form a large leakage clearance, particularly in relatively cold operation. If the clamping force of the mounting bolts is increased over the entire region of the flange portion in order to avoid any leakage clearance being formed, the exhaust manifold is subject to heavy thermal stressing and will undergo thermal cracking.